Electrode assembly for an electron discharge device made from a material having a low carbon content



1966 M. FEINLEIB ETAL 3,293,

ELECTRODE ASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE MADE FROM A MATERIAL HAVING A LOW CARBON CONTENT Filed Jan. 4, 1965 TIME (HOURS) LOW CARBON CATHODE, HIGH CARBON SUPPORT STRUCTURE LOW CARBON CATHODE, LOW CARBON SUPPORT STRUCTURE INVENTORS MORRIS FEINLEIB ROBERT L. LUM

TTORNEY 0.1 lb :00 100E United States Patent C ELECTRODE ASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE MADE FROM A MA- TERIAL HAVING A LOW CARBON CON- TENT Morris Feinleib, Los Altos, and Robert L. Lum, Menlo Park, Calif., assignors to Varian Associates, Palo Alto, Calif, a corporation of California Filed Jan. 4, 1963, Ser. No. 249,350 14 Claims. (Cl. 313-311) This invention relates to electron discharge devices operating under vacuum, and in particular to the cathode structures of such devices.

During the evacuation of an electron discharge device, such as an electron tube, it is customary to subject vari ous parts of the electron tube to a series of heating steps, which aid in achieving a high degree of vacuum and an electron-emitting cathode with good activity and stability. Typically, a tube comprising a cathode coated with alkaline earth emissive compounds can be processed as follows: (a) after the tube is under vacuum, the envelope is first baked to temperatures ranging typically from 350 to 650 C. in order to degas it; (b) the cathode is heated in steps in order to convert the emissive coating to its final form. Typically, the initial coating consists of a mixture of carbonates, which evolve carbon dioxide in the process of thermal conversion to the oxides; (c) the cathode is further heated to temperatures which typically range from 100 to 300 C. above its ultimate operating temperature. This last step results in activating the cathode for emission as well as in further outgassing of the cathode base material and associated structure.

Normally, the two objectives of step (c) requiring confiicting conditions. For purposes of initiating emission, it is desired to keep the activation time as short as possible. The activation step results in the chemical reduction of the emissive coating by activating additives present in the cathode base (typically nickel), yielding free barium. If this step is unduly prolonged, excessive reduction takes place: free barium is vaporized over the tube, causing undesired electrical leakage; also, the supply of activator in the base metal is prematurely depleted, resulting in shorter cathode life. On the other hand, for purposes of gas removal it is desired to prolong the ulti mate heating as much as possible in order to eliminate or at least minimize any long-term gas evolution at cathode operating temperatures. In practice, the combination of time and temperature chosen for step (c) is a compromise dictated by the amount of barium evaporation and cathode deactivation that can be tolerated, the ultimate pressure required, the pumping speed of the vacuum system connected to the tube, and the size of the connections to the pump (also known as vacuum conductance).

In modern tubes built for long life and long reliability, and particularly in many microwave tubes, the requirements for a high vacuum are quite stringent, typically in the range between 1() and torr over the life of the tube. In order to achieve such pressures, long outgassing periods at high temperatures are normally mandatory. Unless this is done, the gas pressure will build up slowly during tube operation, and may result in undesirable conditions of ion bombardment, noise, unstable electrical characteristics, etc. At the same time, in many of the same tubes, the tolerance for barium evaporation and/ or cathode deactivation is very low.

It is toward a solution of the above dilemma that the present invention is directed. Accordingly, it is the principal object of this invention to provide an electron discharge device which has a consistently faster outgassing time, has a minimum outgassing rate at activation temperatures, has a uniform emission level during operation,

and has a consistent, longer life expectancy than heretofore known.

It has been discovered that the amount of residual gas evolved in a vacuum electron tube when the cathode is at its normal operating temperature or above can be drastically decreased by building substantially the whole cathode assembly of metals with a very low carbon content.

More specifically, one feature of this invention is the provision of an electron discharge device in which all parts of the device which operate at or above 650 C., with the possible exception of the tungsten heating filament, have a carbon content not exceeding parts per million (100 p.p.m.), and preferably below 50 p.p.m.

Another feature of the present invention is the provision of a cathode assembly for an electron discharge device made of material, preferably nickel or nickel alloy, having a carbon content not exceeding 100 p.p.m. and preferably below 50 p.p.m.

Other objects and features and a fuller understanding of the present invention may be had by referring to the following description and claims, taken in conjunction with the following drawings in which:

FIG. 1 is a longitudinal cross-sectional view of an electron discharge device of the subject invention;

FIG. 2 is an enlarged fragmentary cross-sectional view of the area delineated in 2--2 FIG. 1;

FIG. 3 is a log-log graph of pressure vs. time for a low carbon button, high carbon support structure and low carbon button, low carbon support structure, cathode assemblies; and

FIG. 4 is a longitudinal cross-sectional view of another preferred embodiment of the subject invention.

Referring to FIGS. 1 and 2 there is shown an electron discharge device embodying the subject invention as, for example, a simple diode tube 11 having an envelope 12, cathode assembly 13, and anode structure 14, shown with an exhaust port 15.

Referring particularly to FIG. 2 the cathode assembly 13 is shown in more detail as having contained within an outer cylindrical metallic shield 16, as for example, nickel or nickel alloy, a flat circular cathode button or base 17. Cathode button or base 17 is supported by an apertured cup member 18 which is spot-welded to the back of cathode button 17 A generally cylindrical tubeshaped member 19 is spot-welded both to cup member 18 and shield 16 so as to support and hold cathode button 17 in axial alignment.

Referring nowto FIG. 1, the shield 16 is supported and held in axial alignment by support wires 20 and a header assembly 21. The cathode button 17 is most often made of nickel or nickel alloy and is coated on its outer surface with a substance (not shown), as for example, a combination barium oxide and strontium oxide, which when heated, is electron-emissive. Heating of the cathode button 17 is accomplished by means of a special wound filament 22 (see FIG. 2), typically made of tungsten wire contained within the space formed by the cathode button 17 and cup member 18. Filament 22 is supported within the cup member 18 by means of insulated filament lead-in wires 23.

It has been realized for some time by those skilled in the art that carbon is an active reducing agent, and that it will react with the emissive alkaline earth oxide coating to produce free barium and carbon monoxide gas. In order to reduce the extent of this reaction and of resultant carbon monoxide evolution, it was decided that the amount of allowable carbon in nickels used in cathode buttons or bases should be limited. The concensus of present expert opinion in this matter is set forth in specification F239-61T of the American Society for Testing Materials (ASTM), dealing with Nickel Alloy Cathode Sleeves for Electron Devices. This specification recognized several grades of Active Alloys and Passive Alloys. For active alloys, which cause a relatively rapid reduction of the emissive coating, the maximum carbon level specified varies between 0.08 and 0.10%. Even for the passive alloys, which are used when a slower reduction of the emitter oxide and a minimum rate of barium evaporation are desired, the maximum carbon level is given as 0.05%. No attempt is made in the above specification to control the carbon level, as long as it stays below the maximum, and this attitude is also reflected in all normal commercially available grades of cathode button or base nickel alloys, which exhibit wide variations of carbon level from one lot to another.

As pointed out above, the evolution of gas from carbon containing cathode buttons or bases has usually been attributed to a chemical reaction between the carbon and the emissive oxide coating. Therefore consideration had been given only to the emitter base (button or sleeve) rather than to the associated structures (heat shields, cathode support structure, etc.). In an article entitled, Thermionic Emission vs. Carbon Diifusion in Nickel Based Oxide Cathodes, by C. W. Caldwell which was reprinted from Advances in Electron Tube Techniques by Pergamon Press in 1961, there was an indication that carbon-containing nickel may cause evolution in a cathode base or button even in the absence of an emissive oxide coating. There was no statement of the carbon level needed for this outgassing to become appreciable. Neither was there any indication that any appreciable gas evolution occurred at normal cathode operating temperatures (typically from 650 to 900 C.).

It was against the above background of the existing art that the benefits of keeping the carbon level below 100 p.p.m. and preferably below 50 p.p.m. in all or substantially all components of a cathode assembly which operate close to or above normal cathode operating temperatures were discovered. It was found that when using nickel with a carbon content below the 50 p.p.m. maximum allowed by the ASTM specification mentioned above, but substantially above 100 p.p.m., considerable gas evolution was noted even near cathode operating temperatures, whether such material is used in the cathode base only, cathode supporting and structural components only, or in the whole cathode assembly. It was also discovered that this was true whether a heater coated with insulating aluminum oxide was used or not.

As a specific example of the fact that the amount of 'carbon present in the cathode button or base has an effect on the length of time it takes to outgas an electron discharge device, and on the amount of harmful outgassing during operation of the electron discharge device, a diode tube as described above was constructed and nickel cathode assemblies of varying carbon content were used in an efiort to determine a critical value for carbon content. In one such test (see dashed line, FIG. 3) a cathode button was used having a carbon content of 32 p.p.m. The rest of the cathode assembly was checked and found to have carbon content of 340 p.p.m. It was heated at 820 C. for nearly five hours while being pumped and the pressure within the tube was reduced from 10- to just below torr. The button was then activated by raising the temperature to 928 for a few minutes and then lowered to 862. During this activation time the pressure within the tube rose to nearly 10 torr and only after fifty-five hours more of outgassing was the previously attained low value of pressure reached. The button was activated again for a few minutes at 971 C. and the pressure rose even higher than previously.

The entire cathode assembly was then replaced by one with a low carbon content (32 p.p.m.), and unexpected and remarkable results were achieved. For example, in one such test (see solid line, FIG. 3) the cathode was heated at 815 C. for just over two hours while'being pumped and the pressure within the tube was lowered from 10 to 10- torr. The button was then activated to 1037" C. for a few minutes and then lowered. During this period of activation the pressure rose, only to a little above 10* torr, but almost as quickly dropped to its original low pressure of 10- torr.

Additional tests were conducted with materials of intermediate carbon contents, and it was established that outgassing can be substantially reduced and average life expectancy can be increased if the entire cathode assembly and all parts of the tube which operate near or above cathode temperature (above 650 C.) are constructed of material having carbon content less than p.p.m. and preferably below 50 p.p.m.

If such low carbon content material is not readily available, stock nickel may be decarburized by any of the well known methods, as, for example, by firing in hydrogen for a length of time which depends upon the thickness of the material.

In different cathode assemblies, for example, in klystron tubes 24 (see FIG. 4) which may incorporate stainless steel cathode assemblies 25 and components, it has similarly been found that the use of stainless steel with a carbon content below 100 p.p.m. yields the benefits of faster outgassing and better tube reliability described herein.

It is to be understood that the use of inconsequential amounts of metal (for example, small lead-in wires) containing over 100 p.p.m. carbon in a cathode assembly does not constitute a departure from the spirit and the scope of the invention.

What is claimed is:

1. An electron discharge device comprising an envelope, electrode elements in said envelope, supporting means in said envelope for said electrodes, said electrode elements including a cathode, said cathode and a preponderance of the parts of said electron discharge device which operate in the range of the cathode operating temperature and above made of material having a carbon content not exceeding 100 parts per million.

2. An electron discharge device comprising an envelope, electrode elements in said envelope, supporting means in said envelope for said electrodes, said electrode elements including a cathode, said cathode and a preponderance of the parts of said electron discharge device which operate in the range of the cathode operating temperature and above made of material having a carbon content less than 50 parts per million.

3. An electron discharge device comprising an envelope, electrode elements in said envelope, supporting means in said envelope for said electrodes, said electrode elements including a cathode, said cathode and a preponderance of the parts of said electron discharge device which operate in the range of 650 C. and above made of material having a carbon content not exceeding 100 parts per million.

4. An electron discharge device according to claim 3 wherein said cathode is coated with an alkaline earth emissive compound.

5. An electron discharge device according to claim 4 wherein said alkaline earth cathode coating is a barium oxide and strontium oxide compound.

6. An electron discharge device comprising an envelope, electrode elements in said envelope, supporting means in said envelope for said electrodes, said electrode elements including a cathode, said cathode and a preponderance of the parts of said electron discharge device which operate in the range of 650 and above made of material having a carbon content less than fifty parts per million.

7. An electron discharge device according to claim 6 wherein said cathode is coated with an alkaline earth emissive compound.

8. An electron discharge device according to claim 7 wherein said alkaline earth cathode coating is a barium oxide and strontium oxide compound.

9. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base, supporting means and shield means made from material having a carbon content not exceeding 100 parts per million.

10. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base, supporting means and shield means made from material having a carbon content less than 50 parts per million.

11. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base, supporting means and shield means made from nickel having a carbon content not exceeding 100 parts per million.

12. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base supporting means and shield means made from nickel having a carbon content less than 50 parts per million.

13. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base, supporting means and shield means made from nickel alloy having a carbon content not exceeding 100 parts per million.

14. A cathode assembly for an electron discharge device including a cathode base, means for supporting said cathode base, shield means surrounding said cathode base, said cathode base, supporting means and shield means made from nickel alloy having a carbon content less than 50 parts per million.

References Cited by the Examiner UNITED STATES PATENTS 3,127,537 3/1964 Horsting 3l3346 3,204,140 8/1965 Kearns 313342 X 3,207,600 9/1965 Hiraistal 252503 FOREIGN PATENTS 69,466 2/1952 Netherlands.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ENVELOPE, ELECTRODE ELEMENTS IN SAID ENVELOPE, SUPPORTING MEANS IN SAID ENVELOPE FOR SAID ELECTRODES, SAID ELECTRODE ELEMENTS INCLUDING A CATHODE, SAID CATHODE AND A PREPONDERANCE OF THE PARTS OF SAID ELECTRON DISCHARGE DEVICE WHICH OPERATE IN THE RANGE OF THE CATHODE OPERATING TEMPERATURE AND ABOVE MADE OF MATERIAL HAVING A CARBON CONTENT NOT EXCEEDING 100 PARTS PER MILLION. 